1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device.
2. Description of the Related Art
A known method of forming impurity regions within a semiconductor substrate includes a photolithography step in which a resist pattern is formed on a semiconductor substrate and an ion implantation step in which impurity ions (that is, ions for forming impurity regions) are implanted into the semiconductor substrate by using the resist pattern. The impurity ions implanted toward a portion of the resist pattern from which the photoresist has been removed reach into the semiconductor substrate, forming an impurity region. On the other hand, the impurity ions implanted toward a portion of the resist pattern in which the photoresist remains do not reach the semiconductor substrate, and thus no impurity region is formed within the semiconductor substrate. As described above, ion implantation is performed using resist patterns corresponding to impurity regions that need to be formed. Hereinafter, the photolithography step and the ion implantation step will be collectively referred to as an “impurity region forming step”. The concentration distribution in the depth direction of the impurity region depends on the conditions for implantation of impurity ions. Accordingly, forming a plurality of impurity regions having different concentration distributions in the depth direction requires as many times of the impurity region forming step as the number of impurity regions.
According to the technique disclosed in Japanese Patent Laid-Open No. 2006-196769, the openings of respective resist patterns used in two times of the impurity region forming step are partially overlapped with each other, whereby three types of impurity regions having different concentration distributions in the depth direction are formed. This reduces the number of times of the impurity region forming step.
With conventional methods, the depths of the peak positions of impurity concentration of impurity regions formed in a single time of the impurity region forming step depend on the ion implantation conditions. Accordingly, in order to form a plurality of impurity regions whose depths from the semiconductor substrate surface to the peak positions are different from each other within a semiconductor substrate, the impurity region forming step needs to be performed on each of the impurity regions. Even when the technique disclosed in Japanese Patent Laid-Open No. 2006-196769 is used, a plurality of impurity regions whose depths to the peak positions of impurity concentration are different from each other cannot be formed with a single time of the ion implantation step.